1. Field of the Invention
The present invention is directed in general to integrated circuit devices and methods for manufacturing and operating same. In one aspect, the present invention relates to the manufacture and use of semiconductor devices having Ion-Sensitive Field Effect Transistors (ISFETs).
2. Description of the Related Art
Ion-Sensitive Field Effect Transistors (ISFETs) and Immunologically Sensitive Field Effect Transistors (IMFETs), hereafter collectively referred to as Chemically-Sensitive Field Effect Transistors (CSFETs), are used for sensing and measuring concentrations of ions and other molecules in fluids. When paired with a standard reference transistor to form a differential pair sensing circuit, the matching electrical characteristics (e.g., threshold voltage Vt) between the CSFET and the standard reference transistor are highly critical. However, the requirement that the CSFET gate be exposed to the fluid (either directly or through a target-sensitive membrane) can alter the threshold voltage of the CSFET, thereby creating a significant mismatch between the CSFET and the standard reference transistor which reduces the sensitivity and accuracy of the differential pair sensing circuit. These mismatch challenges remain, even as CMOS processes are used to fabricate differential pair circuits. For example, known CMOS CSFETs are unsatisfactory in a number of respects, including wide variation in the threshold voltage at a chosen set point (e.g., neutral pH) for the CMOS CSFETs, susceptibility to voltage drift of CMOS CSFETs during operation, and other deleterious effects that arise from the presence of fixed charge within the gate dielectric (and/or passivation layers) of the CMOS CSFETs, as well as process-induced charges within the polysilicon floating gate. As a result, the existing design, operation, and manufacturability of differential pair CSFET sensing integrated circuits is extremely difficult to implement at a practical level.
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for purposes of promoting and improving clarity and understanding. Further, where considered appropriate, reference numerals have been repeated among the drawings to represent corresponding or analogous elements.